Powder detergents, which have been commonly used for domestic purposes, are advantageous in high fluidity, high caking resistance, little dusting, good handling characteristics and high solubility. Most of these powdery detergents are produced by spray drying.
In recent years, the demand for high bulk density granular detergents, which are reasonable from the viewpoint of conservation of resources and convenient for consumers from the viewpoint of easiness in carrying, has been increasing.
However spray-dried detergent particles generally have a bulk density of 0.4 g/cm.sup.3 or below and an average particle size of from 20 to 1,000 .mu.m. Therefore it is difficult to obtain a high bulk density granular detergent directly by conventional spray drying methods.
Known methods for producing high bulk density granular detergents include a method comprising granulating spray-dried detergent particles as disclosed, for example, in JP-A-61-69897 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); a method comprising compressing spray-dried detergent particles and then milling and granulating as disclosed, for example, in JP-A-61-69899; a method comprising mixing, solidifying and breaking-up a detergent material as disclosed, for example, in JP-A-61-76597; a method comprising mixing detergent powders followed by solidifying and multi-step milling as disclosed, for example, in JP-A-63-150398; and a method comprising continuously granulating detergent particles as disclosed, for example, in JP-A-2-232299 (corresponding to U.S. Pat. No. 5,018,671).
However, these known methods suffer from various disadvantages as follows. In the method disclosed in JP-A-61-69897 which comprises mixing, agitating and granulating a spray-dried product, it is very difficult to control the particle size and bulk density of the resulting granular product. In addition, the achievement of a high bulk density is accompanied by a poor yield and, furthermore, the use of the spray-dried product of a low density makes it necessary to use a large-scaled equipment.
In the method disclosed in JP-A-61-69899 which comprises compressing a spray-dried product followed by milling and granulating, a large amount of fine powders are formed during the milling step, which causes problems in qualities and productivity and, further, a uniform particle size can hardly be obtained by this method.
In the method disclosed in JP-A-61-76597 which comprises breaking and granulating a solid detergent material, both of the moisture content of the starting material and the breaking time are restricted. Thus this method is not commonly applicable from the viewpoints of operation characteristics and the composition.
In the method disclosed in JP-A-63-150398 which comprises multi-step milling step where the formation of fine powders is prevented, a solid detergent material is fed into a cutter type mill of a large screen pore size to other ones of smaller screen pore sizes successively when the solid detergent material is milled and granulated. However, the method requires a large equipment to perform the multi-step treatment on an industrial scale.
In the method disclosed in JP-A-2-232299 (corresponding to U.S. Pat. No. 5,018,671) which comprises continuously granulating detergent particles, a distribution in the residence time during the continuous operation makes it difficult to achieve a uniform grain size distribution. Further, this method suffers from some problems in the suppression of the formation of fine powders and in the control of bulk density.